Minimising the environmental impact of well testing has, for some time, been a major issue in the oil industry. In some areas of the world, legislation and taxation upon greenhouse gases produced can double the cost of a well test. The ability to conduct a well test without the necessity to flare the produced hydrocarbons and still obtain the quality and quantity of data required to allow formation to be evaluated correctly would significantly increase the number of tests conducted on a worldwide basis.
Traditional well test operations involves the production and disposal of hydrocarbons creating large quantities of both greenhouse and noxious emissions and the relatively high risk of pollution due to inefficient combustion of the hydrocarbons or accidental spillage.
Several different techniques have been developed to date in an attempt to minimise the environmental impact of well testing. The two techniques which are most commonly used are:
a) downhole pressure and sampling systems, such as Schlumberger's MFT, RFT and MDT tools or Baker's RCI system;
b) close chamber testing such as that developed by Halliburton and used in environmentally sensitive areas, such as the Gulf of Mexico or onshore California.
The amount of information which is obtained by downhole logging systems is limited, primarily due to small volumes which flow from the formation providing samples which can be contaminated by fluids used during the drilling wells and also due to a very small radius of investigation into the reservoir which can lead to the skin effect (formation damage created by the drilling process) having an overwhelming effect on the information obtained. One significant advantage of the aforementioned system over conventional well testing is the ability to determine the vertical permeability of the formation. The close chamber testing minimises the environmental impact of the test but, once again, due to the relatively small volumes of fluid displaced, provides limited data in terms of quality and quantity. In fact, one of the major problems associated with any type of close chamber testing has been resolution of downhole gauges. With relatively small produced volumes, the change and pressure in any normal sized reservoir is very small and until recently with the development of quartz crystal gauges, these pressure changes have been undetectable. This problem combined with the constantly changing skin and flow rate effects during the initial flow period have made evaluation of close chamber data exceedingly difficult and potentially unreliable. Indeed, a significant disadvantage of conventional close chamber systems is the very small volume of fluid which is taken from the formation due to low storage volumes which does not allow uncontaminated pressure volume temperature (PVT) samples to be obtained.